The present invention is related to the field of optical fiber technology and, more particularly, to wavelength division multiplexed (WDM) couplers, and integrated WDM coupler and isolator devices which are highly suitable for fiberoptic amplifiers.
A WDM coupler transfers input signals from a plurality of input information channels to a plurality of output information channels in response to the wavelength of the input signals. A goal for any WDM coupler is that the crosstalk between channels is zero, i.e., that an untargeted output channel is effectively isolated from the signals on a targeted output channel so that none of the signals leak onto the untargeted channel.
Fiberoptic amplifiers boost message signals of one wavelength from the pump signals of another wavelength. These fiberoptic amplifiers, especially erbium-doped fiber amplifiers, are increasingly being used in high-speed fiberoptic transmission links and networks. These types of amplifiers are readily insertable into various points of a network to provide repeater functions, for example, to boost optical signals traveling through many kilometers of optical fibers.
The fiberoptic amplifiers are coupled to lasers which supply the pump signals by wavelength division multiplexed couplers. Since the lasers are susceptible to noise, isolators are also coupled to the amplifier/laser systems to block noise and spurious signals which denigrate the performance of the amplifier. These devices allow a pump laser to be effectively coupled to the fiberoptic amplifier so that a message signal through the fiberoptic amplifier is amplified from the energy supplied from the laser signal.
One partially integrated wavelength division multiplexed coupler and isolator is described in U.S. Pat. No. 5,082,343, which issued Jan. 21, 1992 to D. G. Coult et al. The described device is suitable for coupling to fiberoptic amplifiers. However, the device has several disadvantages. First, the wavelength division multiplexed portion of the device operates inefficiently with undesirably high insertion losses and low isolation losses (large amounts of crosstalk). Secondly, the device is rather large, which has an adverse effect upon the reliability and robustness of the device. Sealing the device, for example, is more of problem with a large device. Furthermore, large size makes the device difficult to insert into various points of a fiberoptic network, as may be desired. Still another disadvantage of the described device is that other desirable components, such as a tap coupler for monitoring signals through the device, must still be linked by fiber splicing. This lowers the performance of the overall system and creates further reliability problems.
The present invention solves or substantially mitigates these problems with a wavelength division multiplexed coupler with high isolation between channels, which additionally is integrable into a compact WDM coupler and isolator device.